Among the conventionally-known airbag apparatus are ones in which a strap (hereinafter referred to as “tether”) for retaining an airbag in a deployed configuration capable of protectively holding or restraining a vehicle occupant is provided within the airbag, the airbag has a vent hole formed therein, the tether is provisionally joined to an inner surface portion of the airbag around the vent hole with the airbag kept in a folded configuration, the provisional joint is released immediately before the airbag is deployed to protectively restrain the vehicle occupant (see, for example, Japanese Patent Application Laid-Open Publication No. HEI-6-107099).
In the airbag apparatus disclosed in the HEI-6-107099 publication, the tether is kept provisionally joined to the inner surface of the airbag around the vent hole until immediately before the airbag is deployed, by gas ejected from the inflator, into a configuration capable of protectively restraining the vehicle occupant. Thus, the vent hole is kept closed with the tether until immediately before the airbag is deployed. Hereinafter, the sufficiently deployed configuration of the airbag, which is capable of protectively restraining the vehicle occupant, will be referred to as “occupant restraining configuration”. Namely, with the airbag apparatus disclosed in the HEI-6-107099 publication, the airbag can be deployed without the gas escaping from the airbag until immediately before the airbag is deployed to the occupant restraining configuration.
Once the airbag is deployed almost to the occupant restraining configuration, the tether is pulled strongly by the airbag so that the provisional joint is canceled. Then, the tether is taken off the inner surface portion of the airbag around the vent hole, so that the vent hole is opened. By causing the gas to escape through the opened vent hole, the inner pressure of the airbag can be adjusted appropriately.
However, because the vent hole is opened immediately before the airbag is deployed to the occupant restraining configuration, the airbag apparatus disclosed in the HEI-6-107099 publication would present the inconvenience that the gas undesirably escapes through the vent hole before the airbag is deployed to the occupant restraining configuration and thus it is difficult to deploy the airbag efficiently and stably. Consequently, with the disclosed airbag apparatus, it is necessary to increase the amount of the gas generated by the inflator to some degree.